Many automobile parts such as chassis and body structures are conventionally produced by press-forming steel sheets having a prescribed strength at room temperature. In recent years, weight reduction of automobile bodies has been desired from the viewpoint of the preservation of the global environment. Thus, there have been continuous efforts to decrease the thickness of steel sheets used for automobile parts by increasing the strength of such steel sheets. However, increasing the strength of a steel sheet is accompanied by a decrease in press-formability of the steel sheet and it is frequently difficult to press-form such a steel sheet into automobile parts with desired shapes.
To remedy this difficulty, United Kingdom Patent Publication No. 1490535 proposes a technique in which a steel sheet that has been heated is press-formed with a mold composed of a die and a punch while being simultaneously rapidly cooled. That technique, called “hot” press-forming, realizes good press-formability of a high strength steel sheet.
However, that hot press-forming involves heating of a steel sheet to a high temperature of about 950° C. before press-forming. As a result, scales (iron oxides) are formed on the surface of the steel sheet. Such scales are peeled off during the hot press-forming and damage the mold or damage the surface of the hot press-formed parts. Further, scales remaining on the surface of the parts deteriorate the appearance or decrease paint adhesiveness. Thus, the surface of such parts is usually descaled by treatments such as pickling or shot blasting. However, implementation of these treatments makes the producing steps complicated and decreases productivity.
Such automobile parts as chassis and body structures require excellent corrosion resistance, but the hot press-formed parts produced through the above treatments exhibit significantly insufficient corrosion resistance due to the fact that they are not provided with an anticorrosion film such as a coating layer.
Thus, there has been a demand for a hot press-forming technique which can suppress formation of scales during heating before hot press-forming and also can increase corrosion resistance of hot press-formed parts. This has led to proposals of steel sheets having a film such as a coating layer on the surface, and hot press-forming methods using such steel sheets. For example, Japanese Patent No. 3663145 discloses a method of producing a hot press-formed part excellent in corrosion resistance in which a steel sheet coated with Zn or Zn-based alloy is hot press-formed so that Zn—Fe-based compound or Zn—Fe—Al-based compound is formed on the surface of the part. Japanese Patent No. 4039548 discloses a hot press-formed article (part) excellent in formability, weldability, and corrosion resistance obtained by heating a galvannealed steel sheet at 700° C. to 1000° C. for 2 to 20 minutes and hot press-forming the steel sheet to form a coating layer containing Fe—Zn solid solution phase on the surface of the article. Japanese Patent No. 4883240 discloses a steel sheet for hot press-forming excellent in paint adhesiveness and perforation corrosion resistance after heating which is manufactured by forming a Zn—Ni coating layer on the surface of a coating layer I mainly containing Ni, and further by forming a compound layer containing, for example, Si, Ti, Al, and Zr thereon.
However, the hot press-formed part described in Japanese Patent No. 3663145 and Japanese Patent No. 4039548 often exhibits low resistance against perforation corrosion (hereinafter, called perforation corrosion resistance) which tends to occur at portions where chemical conversion films or electrodeposited films are not fully formed.
Although a hot press-formed part excellent in perforation corrosion resistance can be obtained using the steel sheet for hot press-forming described in Japanese Patent No. 4883240, a hot-pressed part can be further preferably applied in a part which is subjected to severe corrosive environment, if the perforation corrosion resistance of the hot press-formed part can be further improved, and if sufficient joint corrosion resistance can be additionally achieved.
It could therefore be helpful to provide a steel sheet for hot press-forming which is necessary to produce a hot press-formed part excellent in perforation corrosion resistance and joint corrosion resistance in a severe environment, a hot press-formed part produced using the steel sheet for hot press-forming, and a method of producing a hot press-formed part using the steel sheet for hot press-forming.